Autoimmune and inflammatory diseases affect more than fifty million Americans. As a result of basic research in molecular and cellular immunology over the last ten to fifteen years, approaches to diagnosing, treating and preventing these immunological based diseases have been changed forever. By dissecting the individual components of the immune system, those cells, receptors and mediators which are critical to the initiation and progression of immune responses have been, and continue to be, elucidated. Crystallographic analysis of proteins encoded in the major histocompatability complex, identification of an antigen-specific T cell receptor, and development of a basic understanding of the complex cytokine network have all contributed to a revolution in immunology. Various immunosuppressive agents have proved to be useful in the prevention of transplantation rejection and in the treatment of autoimmune diseases such as rheumatoid arthritis, nephritis, uveitis, thyroiditis, and early stage of insulin dependent diabetes mellitus, systemic lupus erythematosus, psoriasis and inflammatory bowel disease.
The immune system when operating normally is involved in precise functions such as recognition and memory of, specific response to, and clearance of, foreign substances (chemical and cellular antigens) that either penetrate the protective body barriers of skin and mucosal surfaces (transplanted tissue and microorganisms such as bacteria, viruses, parasites) or arise de novo (malignant transformation). The arsenal of the immune response is composed of two major types of lymphocytes that are either B-lymphocytes (B cells, responsible for producing antibodies which attack the invading microorganisms) or the T-lymphocytes (T cells, responsible for eliminating the infected or abnormal target cells) in cooperation with macrophages. The cascade of principal events in the immune system is more fully described by I. Roitt, J. Brostoff and D. Male in "Immunology", 3rd edition, Mosby, 1993 which is herein incorporated by reference, and may be summarized as follows.
The response is initiated by the interaction of an antigen with macrophages and surface antibodies on B cells. The macrophages ingest and process the antigen. The activated macrophages secrete interleukin-1 (IL-1) and tumor necrosis factor (TNF), and display the processed antigen on the cell surface together with a major antihistocompatibility antigen. Both IL-1 and TNF initiate a number of processes involving inflammation. Also, IL-1 induces proliferation of B cells and synthesis of antibodies. But more importantly, IL-1 activates T cells which release a series of lymphokines including interleukin-2 (IL-2) that activate the proliferation of T cells and cytotoxic lymphocytes. In autoimmune diseases, the system is unable to distinguish between "non-self" antigen and "self" antigen and will start to produce autoantibodies or autoreactive T cells which attack the normal components of the body.
Each element in the cascade of the immune response may be considered as a potential site for pharmacological intervention. For example, adrenocorticosteroids act in the first stages of the immune response, interact with the macrophages and, inhibit the synthesis and release of IL-1. Other immunosuppressive agents used in the treatment of autoimmune diseases have been identified, such as azathioprine and methotrexate for rheumatoid arthritis, cyclophosphamide for nephritic conditions of immune origin, and cyclosporin for rheumatoid arthritis, uveitis, early onset insulin dependent diabetes mellitus, psoriasis, nephritic syndrome and aplastic anemia.
In addition, immunosuppressive agents have proved to be useful in preventing and treating organ transplantation rejection that may occur in allograft transplantation. In allograft transplantation one person donates an organ to a genetically disparate individual while in xenograft transplantation an organ of one species is transplanted into a member of another species. In those cases, the use of cyclosporin has shown a real improvement in the condition of the person receiving the organ. However, the therapeutic index of the available immunosuppressive drugs is narrow, none of the drugs are completely effective and their use has been limited by severe toxicity.
It has been established that various extracts and components of the extracts of Tripterygium wilfordii Hook F, an herbal plant from the Celastraceae family that is grown mainly in the southern part of China, are useful as immunosuppressive agents. Zhang, et al., Shanghai Yike Da ue Xuebao, 13(4), 267 (1986), have characterized T. wilfordii as comprising at least six different diterpenoids, including triptonide, triptolide, triptophenolide and triptonolide. More specifically, P. E. Lipsky, et al. in WO 91/13627, published Sep. 19, 1991, disclosed that extracts or components of the extracts of Tripterygium wilfordii Hook F are useful in suppressing autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosus and psoriasis. Triptolide has been reported by Yang, et al, Int. J. Immunopharmac., 14, 963 (1992) and Yang, et al., Int. J. Immunopharmac., 16, 895 (1994), to suppress lymphocyte proliferation and skin allograft rejection. In addition, Jin and Wiedmann, in WO 94/26265, published Nov. 24, 1994, disclosed a composition wherein an additional component of Tripterygium wilfordii Hook F, purified 16-hydroxytriptolide, is administered in conjunction with another immunosuppressive agent, such as cyclosporin A, FK506, azathioprine, methotrexate, rapamycin, mycophenolic acid, or a glucocorticoid. The above composition was disclosed as providing an increase in immunosuppressive activity relative to the sum of the effects produced by 16-hydroxytriptolide or the other immunosuppressive agent used alone. This allowed for greater immunosuppressive activity with reduced toxicity in immunosuppressive therapy, such as in therapy for transplantation rejection and autoimmune disease. P. E. Lipsky, et al. disclosed in U.S. Pat. No. 5,580,562, published Dec. 3, 1996, a Tripterygium wilfordii Hook F preparation which has an improved LD50 in mice, an improved therapeutic activity:toxic index ratio and a lower amount of triptolide as compared to previous preparations.